Systems and methods herein generally relate to processing and printing optically obtained image files, and more particularly to document background suppression.
While historically images were captured using photographic film, modern digital devices (such as scanners, cameras, etc.) capture images as pixels electronically, and most such digital devices capture images in a color space referred to as RGB, or red-green-blue color space. The RGB color space is utilized by image capture devices because such devices commonly includes pixel sensors that distinguish between the red, green, and blue colors.
When processing images captured by digital devices to make color corrections, remove undesirable artifacts, improve image resolution, suppress background, prepare images for printing or display, etc. (which is sometimes referred to as image path processing) it is often useful to first convert the electronic images into a color space that has a broader or greater spectrum, such as the L*a*b* color space.
The L*a*b* color space has an L dimension for lightness and a and b that are color-opponent dimensions, and are based on nonlinearly compressed coordinates. The L*a*b* color space includes all perceivable colors, which means that its gamut exceeds those of the RGB and CMYK color spaces, but the L*a*b*-color space is device independent, which means that the colors are defined independent of their nature of creation or the device they are displayed on.
Printing devices generally operate in a lower gamut color space, such as the RGB or CMYK color spaces, that are named based on the colors of the marking materials (e.g. inks, toners, etc.) used; such as red, green, and blue (RGB); or cyan, magenta, yellow, and black (CMYK).
Background suppression is a useful function provided by the image path in copiers and multi-functional systems that removes or unifies the color of the background in the digitally acquired image. Thus, the “background” is the color of the paper or other print media of the item that was scanned to create the digital image, and background suppression removes the background or makes the background uniform to make the electronic image appear more uniform and consistent.
One approach of performing background suppression is to apply a linear transformation (gain and offset) on the luminance channel of the image, augmented by some chrominance adjustment for background pixels. One side effect of such approaches can be an undesirable color appearance change in the image. Some background suppression methods improve automatic dynamic range adjustment; however, such methods add processing time, which impacts system performance. Additional methods of background suppression clip pixel values above a determined white point (where the pixel is set to white if the input value is above the determined white point); however, such approaches can produce visible switching artifacts. What is desirable is a computationally efficient background suppression method that does not introduce unwanted color appearance change and does not produce switching artifacts.